The present invention relates to an optical memory element which optically records, reproduces and/or erases information.
It is the recent trend in the field of information processing technology that greater magnetic memory capacity is strongly demanded as information is diversified and increases in volume. To meet this demand, an optical recording system using an optical recording medium has been developed and put into practical use, replacing the conventional recording system. However, an erasable and reloadable optical disc for practical use has not yet been realized at present and is expected to be developed in the immediate future.
There are various kinds of erasable and reloadable optical memories: one with a magneto-optical medium or based on the so-called magnetic Kerr effect, one based on the photo-darkening phenomenon of amorphous chalcogenide and one based on the light reflectance variation effected by the recording medium phase change. The most promising type among these is the system with a magneto-optical medium.
Some of the optical magnetic media used include earth metal-transition metal amorphous alloy. They are defective in that the Kerr rotation angle, which is vital for the signal reading performance, is small. A number of methods have been tried in an attempt to increase the magnetic force-rotation angle of a magneto-optical medium. One of them is to use a multi-layer film having a reflecting film structure thereby increasing the apparent Kerr rotation angle and improving the signal quality. In this method, a part of incident light is reflected by the magnetic film due to the result of the Kerr effect. The rest goes through the magnetic film and is reflected by the reflector film on the other side of the magnetic film back into the magnetic film due to the result of the Faraday effect in the magnetic film. The combination of the Faraday effect and the Kerr effect causes the apparent Kerr rotation angle of the total incident light to increase, thus raising the C/N ratio.
FIG. 2 is a structural drawing of the conventional magneto-optical disc, in which 1 is a glass or plastic plate, 2 is an AlN film, 3 is a GdTbFe film, 4 is an AlN film and 5 is an Al film.
In the magneto-optical disc of the above structure, the reflector film desirably has as large reflectance as possible. Therefore, Al, Cu, Au or Ag, which have a high reflectance in the semiconductor laser wavelength range, are used as reflector film materials. These elements, however, provide large thermal conductivity in general and therefore impair the recording sensitivity of the magneto-optical disc, since heat absorption by the reflector film during recording by laser is largely due to the high thermal conductivity of Al, larger laser power is required for recording, which inversely affects the recording sensitivity of the magneto-optical disc.